1. Field of the Invention
This invention relates to a multi-cavity mold, a method of manufacturing the mold and molding control method using the mold. In the present invention, a multi-cavity mold is taken to include single cavity multi-gate molds as well. The reason is that both types of molds share a common concept in terms of having a plurality of hot runners and sub-runners.
2. Description of the Related Art
Multi-cavity molding is carried out using a multi-cavity mold. A multi-cavity mold is formed to include a plurality of cavities each having a shape the same as that of the article to be molded (in general, the shape of the cavity is formed to be larger than the article to be molded in anticipation of the amount of shrinkage of the resin molding material). In order to fill the plurality of cavities with a molten resin which flows into the mold along a single channel upon being injected from an injection molding machine, the mold is formed to have a plurality of first runners (main runners) branching from the single channel and corresponding to the plurality of cavities, as well as a plurality of second runners (sub-runners) each having two ends, wherein one end leads to the terminus of a respective one of the first runners and the other end serves as a gate facing the corresponding cavity.
In order to obtain molded articles free of such defects as warpage and without any variance in dimensions between cavities in multi-cavity molding, it is required that all of the cavities be filled with the molten resin simultaneously while such defect-causing phenomena as short shot (insufficient resin) and over-packing (excessive resin, which gives rise to burrs) are avoided. In actuality, however, the cavities and hot runners (the first and second runners) exhibit dimensional variance because of the manufacturing process, and therefore the fluidic resistance of the molten resin in the cavities differs from one cavity to another. For this reason, it is impossible to fill all of the cavities with molten resin up to the end of each simultaneously and uniformly.
Accordingly, in the conventional mold for multi-cavity molding, the practice is to control the temperature of heaters with which the hot runners are provided and change the temperature of the molten resin that will fill each cavity, thereby filling the cavities with the molten resin simultaneously.
This temperature control as practiced in the prior art will be described in greater detail. The multi-cavity mold is provided with a single first heater shared by the plurality of first runners, and with independently temperature-controllable second heaters for individual ones of the plurality of second runners. The second heaters are mainly for controlling finish at the gates. In the prior art, the second heaters are used also for the purpose of controlling the amount of resin filling the cavities.
As set forth above, the fluidic resistance of the molten resin differs from one cavity to the next since the hot runners and cavities exhibit dimensional variance. When it is attempted to fill all of the cavities with molten resin at the same temperature, the cavities for which the fluidic resistance is relatively small may be filled up to their ends with the molten resin, but the cavities of a comparatively large fluidic resistance are not filled with the molten resin sufficiently. Accordingly, the temperature of the molten resin which fills the cavities of a relatively large fluidic resistance is raised by the second heaters. The higher the temperature of the molten resin, the higher the fluidity thereof and therefore the easier it is to fill the cavities. By thus controlling the temperature of the second heaters, all of the cavities are filled with the molten resin in a well-balanced manner.
However, as mentioned above, the second heaters are for controlling the temperatures of the gates, and gate temperature is intimately concerned with gate finish and moldability. As a consequence, when different temperatures are applied to the gates to fill the cavities with molten resin in balanced fashion, certain problems arise, which will now be described.
From the point of view of gate finish, the resin at a gate is melted if the gate has a temperature that is too high. As a result, resin in the form of a thread remains at the gate of the molded article. This is a defect-causing phenomenon referred to as "strings". On the other hand, when a gate has a temperature that is too low, the resin solidifies at the gate and the mark of the gate left on the molded article defines a convex shape. This is a defect known as "high gate".
In terms of moldability, the fact that the resin at a gate is melted when the gate temperature is too high results in some of the resin flowing out of the gate opening after the mold has been opened and the molded article extracted. This is a phenomenon referred to as "drooling". When a gate has a temperature that is too low, the resin solidifies at the gate and impedes injection of the resin.
Thus, with mold temperature control in multi-cavity molding according to the prior art, it is difficult to reconcile good balance in filling the plurality of cavities with resin, satisfactory gate finish and good moldability. Controlling the distribution of molten resin to a plurality of cavities and controlling the gates by using only one type of heater (the second heaters) is itself unreasonable to start with.
Even if these two types of control can be reconciled to some degree, a problem which arises is that the molded articles will develop a disparity in terms of dimensions and weight if there is a large difference in the temperature of the introduced resin from one cavity to another. The reason for this is that when the temperature of the molten resin differs from one cavity to another, a difference is produced in the amount of resin with which the cavities are filled or supplied during the resin injection step or a dwell step (wherein dwell refers to a process in which a constant pressure is applied after the injection step in order to prevent a situation wherein the desired shape and dimensions are not obtained due to shrinkage caused by cooling of the resin charged into the cavities in the injection process). Another reason is that the degree of shrinkage when the resin solidifies differs depending upon temperature.
A mold has been proposed in which the second runners are provided with two heaters (second and third heaters) the temperatures of which can be controlled independently [for example, see the specification of Japanese Patent Application Laid-Open (KOKAI) No. 63-236615]. However, even in this mold the resin used in order to balance the amount of resin charged into the cavities is merely part of the resin residing in the second runners. Since the molten resin residing in the plurality of first runners is maintained at a substantially uniform temperature by the common heater, the fluidity of the resin at these portions is substantially uniform. Therefore, the amount of resin fill is substantially governed by the temperature of the resin in the second runners and it is difficult to balance the amount of fill. If the temperature difference between first and second runners is enlarged in order to balance the amount of fill, the molded articles will develop a variance in terms of dimensions and weight, as set forth above.
Furthermore, in a case where the amount of resin which collects in the second runners is not enough resin necessary for a single molding operation, all of the resin that has collected in the second runners and some of the resin that has collected in the first runners is charged into the cavities. Therefore, if there is a difference in temperature between the second runners and the first runners, then there is a difference in the degree of shrinkage at one part of a molded article from that at another part thereof and therefore the article develops a defect wherein its shape becomes distorted at solidifying.